In the past decades, lithium-ion batteries (LIBs) have been widely utilized in various applications especially consumer electronics because of their superior energy density, long cycle life and discharging capability. LIBs generally include an anode, an electrolyte, and a cathode that contains lithium in the form of a lithium-transition metal oxide, such as LiCoO2, LiNiO2 and LiMn2O4.
Currently, LIBs mostly utilize metal oxides as cathode material with LiCoO2 as the most popular and commercially successful representative. However, due to the intrinsic material properties of this cathode material, besides the toxicity and high material cost of cobalt, further enhancement of performance of LIBs is also limited. LiNiO2 is characterized for its high specific capacity up to 180 mAh/g. But its application is limited to experimental research because of difficulties in synthesis and safety concerns due to thermal runaway reaction. LiMn2O4 has been considered as a promising cathode material due to its advantages of high stability and low cost. However its low charge capacity and inferior cycling performance, especially under high temperatures, limit the application of this material to small electrokinetic cells.
In recent years, a multi-element lithium transition metal oxide (LNMC) such as ternary transition metal oxides in the form of Li[NixMnyCo1−x−yO2]O2 has been proposed to replace LiCoO2. LNMC adopts the α-NaFeO2 type structure and can be regarded as the partial substitution of Ni2+ and Mn4+ (1:1) for Co3+ in LiCoO2. This multi-element transition metal oxide as cathode material is expected to leverage merits of each component material and might even prevail in the overall performance, because of the synergy effect of the three transition metal ions and the flexibility of composition. Therefore, LiCoO2 is gradually replaced by the ternary transition metal oxides especially in applications that require high power output.
However, in the case of using lithium ternary transition metal oxide (LiNixMnyCo1−x−yO2) as a cathode active material, there are still some drawbacks in the Li-ion batteries to be improved, for instance, phase transformation, safety issue about thermal instability, and deterioration of capacity after repeated charge and discharge cycles.
Different attempts have been made to solve the problems and improve the performance of LiNixMnyCo1−x−yO2 as cathode material, including particle size control (see Shaju et al., Macroporous Li(Ni1/3Co1/3Mn1/3)O2: A high-power and high-energy cathode for rechargeable lithium batteries, Advanced Materials, 2006, 18, 17, 2330), lattice doping (see Hong et al., Nano SIMS characterization of boron- and aluminum-coated LiNi1/3Co1/3Mn1/3O2 cathode materials for lithium secondary ion batteries, Journal of Applied Electrochemistry, 2011, 42, 1, 41) and surface modification (see Song et al., Enhanced electrochemical and storage properties of La2/3−xLi3xTiO3-coated Li[Ni0.4Co0.3Mn0.3O2], Electrochimica Acta, 2011, 56, 20, 6896). The coated cathode reported by Song et al. demonstrated an enhanced rate capability, discharge capacity, thermal stability and cyclic performance through structural stabilization and the overall performance is generally more effective than particle size control and lattice doping. However, this method is not suitable for industrial scale-up for producing the coated cathode on a large scale. Besides, the cycle life using this cathode material so far is less than 50 cycles in terms of 20% loss of its initial capacity, which is insufficient for many intended applications such as portable electronics and electric vehicles.
U.S. Pat. No. 7,678,503 B2 describes a method of modifying a layered oxide with a metal oxide by dispersing a (1−x)Li[Li1/3Mn2/3]O2.xLi[Mn0.5−yNi0.5−yCo2y]O2 composition in a metal salt precursor solution. Ammonium hydroxide is added to the metal salt precursor solution to precipitate a metal hydroxide. The layered oxide containing the metal hydroxide is then heated to obtain a surface modified layered oxide. The surface modification offers the advantage of improving the cycling capacity retention. However, the cycle life using this cathode material so far is less than 200 cycles, and thus still insufficient for actual applications.
U.S. Pat. No. 8,883,352 B2 discloses a method for producing a surface modified lithium-containing composite oxide represented by LiwNxMyOzFa, wherein N is at least one element selected from the group consisting of Co, Mn and Ni, and M is at least one element selected from the group consisting of Al, Zn, Sn, an alkaline earth metal element and a transition metal element other than Co, Mn and Ni. The surface modified lithium-containing composite oxide shows improved capacity and thermal stability. However, this surface modified lithium-containing composite oxide has no sufficient performance from the viewpoint of the battery performance especially the durability for charge and discharge cycles.
In view of the above, there is always a need to develop a method for preparing a surface modified ternary transition metal oxide as a cathode material for lithium-ion batteries with high charging/discharging capacity, improved safety and excellent cycling durability under fast charge/discharge conditions.